Magnetic fields are often used in the production or testing of articles. For example, magneto-resistive and magneto-optic heads, which are used to read and write data on disk drives, are generally tested while placed in a magnetic field. It is important to test such heads to ensure that a defective head is not installed within a disk drive. Moreover, to increase throughput, it is desirable to test for defective heads early in the production cycle. Improvements to testers that use magnetic fields are desired.